It is well known that a notebook computer is additionally equipped with an display port besides a liquid crystal display (LCD) monitor. The display port is used for being connected with an external display device using a D-sub connector to allow a user to make a presentation or give a lecture. Generally speaking, the external display device may be a projector, a LCD monitor or a CRT monitor.
Usually, the notebook computer may switch an image via hot keys on the keyboard. That is, the image may be selectively displayed on only one of the LCD monitor of the notebook computer and the external display device, or both of them. Thus, after the D-sub connector of the external display device is connected to the notebook computer, the image may be switched by pressing the hot keys on the keyboard. After the presentation or the lecture is finished, the image may be switched to the LCD monitor of the notebook computer by pressing the hot keys on the keyboard again. Afterwards, the D-sub connector of the external display device is removed.
Nowadays, a notebook computer in a specific form may actively detect whether the D-sub connector of the external display device is removed and automatically switch the image to the LCD monitor of the notebook computer. Thus, the user does not need to use the hot keys on the keyboard to complete switching.
The conventional detecting method for detecting whether the external display device is disconnected is realized by software. Two pins of the pins in the D-sub connector are used as an inter-integrated circuit (I2C) via which commands or parameters may be transmitted between the notebook computer and the external display device.
That is, when the external display device is connected to a notebook computer using the D-sub connector, the central processing unit (CPU) of the notebook computer may execute an interrogation program periodically (for example, it may execute the interrogation program per 0.5 seconds), and the commands and the parameters are transmitted between the notebook computer and the external display device via the I2C. When the external display device responds to the commands or parameters, the CPU may know that the D-sub connector of the external display device is still connected with the notebook computer. On the contrary, when the external display device does not respond to the commands or the parameters, the CPU knows that the D-sub connector of the external display device is removed. Then, the notebook computer switches the image to the LCD monitor of the notebook computer automatically, and the hot keys are not used.
When the D-sub connector of the external display device is connected to the notebook computer, the CPU of the notebook should execute the interrogation program periodically to know the connecting situation of the external display device, which reduces the whole performance of the notebook computer.